Many electronic displays use a backlighting system. FIG. 1 is a partial perspective view showing two elements of a backlighting system: a backlight diffuser or waveguide 100, and a light source 101. In a typical backlighting system, diffuser 100 is arranged behind a display screen (not shown) to direct light from light source 101 toward the display screen. Light source 101 is located near a wide end of diffuser 100, which tapers in width as it extends across the display screen. Light 102 enters the wide end of diffuser 100, and light-scattering centers 103 in diffuser 100 re-direct light 102 toward the display screen in order to concentrate the light energy there. This kind of arrangement is useful in thin, flat display systems, such as are used in, for example, laptop or “notebook” computers, among other reasons because it economizes on space.
FIG. 2 shows diffuser 100 and light source 101 as they might be used in conjunction with other parts of one kind of conventional electronic display system. FIG. 2 is an orthogonal view of the “top” of the system. In FIG. 2, light 102 from light source 101 is re-directed by diffuser 100 through vertical polarizer 200, a first glass member 201, a liquid crystal spatial light modulator 202, color absorption filters 203, a second glass member 204, and horizontal polarizer 205. The light passing through horizontal polarizer 205 (represented in FIG. 2 by the arrows numbered 206-208) is typically what a user of the display system sees.
The operation of elements 200-205 is as follows. Light 102 from light source 100, and consequently light 102 directed by diffuser 100 toward vertical polarizer 200, is typically “unpolarized” or “randomly polarized,” in that it comprises both vertically polarized light and horizontally polarized light. Vertical polarizer 200 blocks horizontally polarized light in light 102, allowing only vertically polarized light in light 102 to pass through. Next, elements 201-204 further process the vertically polarized light allowed through by vertical polarizer 200. Elements 201-204 constitute one possible embodiment of a “TFT matrix” (thin film transistor matrix) such as is used in many liquid crystal displays. Glass member 201 may be covered with an ITO (indium tin oxide) film which may be used to apply electric fields to liquid crystal in liquid crystal spatial light modulator 202. By application of the appropriate electric field, the orientation of the liquid crystal can be changed to cause the polarization of light that enters the liquid crystal to be changed. That is, when an electric field in one direction is applied to the liquid crystal, the orientation of the liquid crystal will cause vertically polarized light entering one side of the liquid crystal to exit the other side as horizontally polarized light. On the other hand, when an electric field in another direction is applied to the liquid crystal, the orientation of the liquid crystal will have no polarization effect on entering light. In other words, vertically polarized light entering the liquid crystal will exit as vertically polarized light.
The liquid crystal corresponds to pixels. Thin film transistors on glass member 204 control whether the electric field applied to liquid crystal corresponding to a given pixel is changed or not. Horizontal polarizer 205 will block vertically polarized light, allowing only horizontally polarized light to pass through. Thus, by appropriately switching transistors, the light that reaches a viewer of a display screen can be controlled. More specifically, by appropriately switching the transistor of a given pixel, vertically polarized light passed through by vertical polarizer 200 will be changed to horizontally polarized light, and be passed through by horizontal polarizer 205, to reach a viewer. On the other hand, by appropriately switching the transistor of a given pixel, vertically polarized light passed through by vertical polarizer 200 will remain vertically polarized, and be blocked by horizontal polarizer 205.
Color absorption filters 203 control what wavelengths of light are allowed to pass through to a viewer. Thus, for example, light 206 represents a wavelength corresponding to a red color, light 207 represents a wavelength corresponding to a green color, and light 208 represents a wavelength corresponding to a blue color. Using the above-described polarization techniques, the number and intensity of outputs from pixels can be controlled so as to, in combination with color absorption filters 203, produce any desired image in terms of the aggregate effect on the viewer.
As noted earlier, the foregoing arrangement is useful in thin, flat displays because it economizes on space. That is, light from a typically thin, elongated light source such as a fluorescent tube or LED (light-emitting diode) bar can be distributed by a diffuser across a comparatively wide area. However, inefficiencies exist in the above-described system. For example, because approximately half of randomly polarized light is horizontally polarized, vertical polarizer 200 absorbs approximately half of the energy in light 102 directed toward it by diffuser 100. This means that a significant amount of the power applied to generate light source 100 is wasted. Consequently, other systems may be power-starved, or power sources must be made physically larger to compensate. Such physically larger power sources are especially undesirable in applications that use thin displays, since space is typically at a premium.
An approach is needed to address the above concerns.